The present invention relates to a sputtering target for forming an indium-tin oxide layer as a transparent electrode for liquid crystal displays, thin-layer electroluminescence displays, etc., and a method for producing such a sputtering target.
Since an indium-tin oxide layer composed of indium oxide (In.sub.2 O.sub.3) doped with tin oxide (SnO.sub.2) has high transparency and electric conductivity, it is widely used for displays such as liquid crystal displays, thin-layer electroluminescence displays, etc., and conductors connected to anti-icer heaters for window glasses of air planes, etc.
Such an indium-tin oxide layer is usually formed by a sputtering method, an electron beam deposition method, a CVD method, etc. Among them, the sputtering method for forming an indium-tin oxide layer by sputtering a target constituted by a sintered body of indium oxide and tin oxide with an inert gas ion is mostly utilized because it needs a simpler apparatus than any other methods.
The sintered target was conventionally produced from a mixture of In.sub.2 O.sub.3 powder and SnO.sub.2 powder, but such a sintered target posed various problems. One of such problems is an abnormal discharge phenomenon during the sputtering. If this abnormal discharge phenomenon takes place, the resulting sputtered layer does not have a good uniformity, and if sputtering is continued while leaving the abnormal discharge phenomenon to occur, the sputtering surface of the target would have black spots which serve to increase a thin-film resistance of the resultant sputtered layer.
In view of this problem, it was proposed by Japanese Patent Laid-Open No. 4-160047 to provide an indium-tin oxide sputtering target consisting essentially of indium, oxygen and 3 weight % or more of tin and having a relative density of 80% or more, a tin content measured by a linear analysis with an electron probe X-ray microanalyzer (EPMA) being within the range of 0.8-1.2 times an average content thereof; the sputtering target having a surface resistance of 1 m.OMEGA./cm.sup.2 or less; an integrated intensity of an X-ray diffraction peak of a (1,1,0) plane of an SnO.sub.2 phase being 0.5% or less of that of a (2,2,2) plane of an In.sub.2 O.sub.3 phase. This sputtering target is aimed at dissolving the above problem by reducing the amount of the SnO.sub.2 phase. Incidentally, the drastic reduction of the SnO.sub.2 phase can be achieved by heat-treating the oxide mixture at 1350.degree. C. or higher, preferably 1400.degree.-1550.degree. C. for 10 hours or more so that the SnO.sub.2 phase and the In.sub.2 O.sub.3 phase fully react with each other.
Japanese Patent Laid-Open No. 4-293769 discloses a sintered indium-tin oxide target for sputtering having a relative density of 80-100%, and a grain size of 5-30 .mu.m, at least surfaces of grains being free from (In.sub.0.6 Sn.sub.0.4).sub.2 O.sub.3. This target is aimed at forming a transparent electrode layer having a low resistance by a low-temperature sputtering, and it can be formed by sintering a molded indium-tin oxide powder at 1450.degree.-1600.degree. C., and heat-treating it at 1000.degree.-1300.degree. C. in an oxygen-containing atmosphere. In the heat treatment in an oxygen-containing atmosphere, (In.sub.0.6 Sn.sub.0.4).sub.2 O.sub.3 disappears by thermal decomposition.
In both of Japanese Patent Laid-Open Nos. 4-160047 and 4-293769, phases detrimental to indium-tin oxide sputtering targets are effectively reduced. However, both methods of these references need a heat treatment process in addition to the sintering process, making them disadvantageous in productivity and cost. Also, in the method of Japanese Patent Laid-Open No. 4-160047, the SnO.sub.2 phase is reduced but the reduction of the (In.sub.0.6 Sn.sub.0.4).sub.2 O.sub.3 phase is not achieved. In the method of Japanese Patent Laid-Open No. 4-293769, the reduction of the (In.sub.0.6 Sn.sub.0.4).sub.2 O.sub.3 phase is achieved, but the SnO.sub.2 phase is not reduced.